1. Field of the Invention
The present invention relates generally to devices and systems for augmenting cardiac output, and specifically to intra-aortic cardiac assist pumps.
2. Description of the Prior Art
Intra-aortic and intra-ventricular cardiac assist devices are well known in the art. These devices are generally used to reduce the heart's work load after insult or surgery. They may also be used to increase blood flow from the left ventricle of the heart into the aorta in cases of insufficient cardiac output due, for example, to acute or chronic heart ailments or to interference with normal cardiac function during surgery.
Cardiac assist devices fall into two basic categories, those comprising an external pumping chamber which remains outside the body during the entire course of the therapy (extracorporeal) and those comprising an internal pumping chamber which remains inside of the body (intracorporeal). One major drawback to an internal pumping chamber is that it requires extensive surgery for implantation and removal of the device.
Devices with external pumping chambers also have a number of drawbacks. U.S. Pat. No. 4,014,317, which is incorporated herein by reference, describes a cardiocirculatory assist cannula with a an external balloon pump and cardiac pacing electrode. The cannula is inserted percutaneously through the aorta so that its distal end is inside the left ventricle of the heart. During systole, inlet valves on the cannula inside the left ventricle remain open, and the contraction of the ventricle forces blood to flow into the cannula. Then, during diastole, the blood flows out, into the aorta, through one or more outlet valves along the cannula downstream from the inlet valve. A gas-filled chamber, similar in function to an Intra-Aortic Balloon Pump (IABP), is connected to the cannula external to the patient and downstream of the outlet valves. The balloon is typically inflated during diastole and deflated during systole, to assist in perfusion of the coronary arteries. The cannula has a long and narrow shape which presents a significant blood flow restriction, and thus, limits the effective stroke volume of the device. Accordingly, the device is of limited usefulness in augmenting the blood output of a weakened or failing heart.
U.S. Pat. No. 4,906,229, which is also incorporated herein by reference, describes a high-frequency transvalvular axisymmetric external blood pump. The pump includes a small internal volume in a stiff barrel, which may be alternately expanded and reduced by pneumatic or hydraulic pressure which is exerted via a flexible membrane radially surrounding the volume. The volume has intake and outlet ends, with one-way axial valves at both of the ends, so that blood can flow only from the heart into the aorta. The pump is connected via the one-way intake valve to a cannula, which is inserted into the left ventricle of the heart through the aortic valve. When the internal volume is expanded, blood flows into the pump from the ventricle. The volume is then reduced, and the blood is ejected into the aorta through the outlet end. This pump is designed to operate at a frequency of 600 to 1,000 cycles per minute. Since the stroke volume of the pump is typically only about 3-5 cc, these high cycle rates are needed in order to provide adequate perfusion.
A major drawback of the prior art extracorporeal intra-aortic cardiac assist devices involves an inherent design limitation of said devices. The prior art extra-corporeal intra-aortic cardiac assist devices pump blood out of the left ventricle, through a cannula, and into a downstream portion of the artery. There is a desire to make the inner diameter of the cannula as large as possible so as to allow for the greatest possible blood flow rate through said cannula. There is also a desire, however, to make the outer diameter of the cannula as small as possible to ease its insertion into the artery and so as not to substantially reduce the blood flow in the artery around the cannula. As a result of these competing design goals cannulae are generally designed large enough to accommodate only 20-40 cc of blood per heart cycle. The average patient, however, requires approximately 80-100 cc of blood per heart cycle for full blood flow support.
Aside from the internal/external pumping chamber distinction, cardiac assist devices are also categorized according to their pump drives, which are either continuous or pulsatile flow. In the Hemopump Cardiac Assist System, distributed by Johnson & Johnson Interventional Systems, a cannula containing a special, miniature rotor pump mechanism is inserted into the aorta. The pump is driven by a drive unit outside the body, to pump blood continuously from the aorta into the rest of the arterial system, thereby supplementing to some degree the heart's natural output rate. A system of this type is similarly described in U.S. Pat. No. 5,092,844, which is incorporated herein by reference. A drawback of this system is that the outer diameter of the pump, and accordingly the pump's output, is limited due to the need for insertion through the femoral artery. A further drawback of this system, and of continuous-flow devices in general, concerns the belief that pulsatile pumps provide more effective long-term support than continuous-flow devices since they approximate more closely the natural pump action of the heart.
One of the best-known and most widely-used intra-aortic pump systems is the Intra-Aortic Balloon Pump (IABP), comprising a catheter, having an inflatable balloon at its distal end, which is inserted through an artery into the aorta. The balloon is alternately inflated and deflated by an external pump drive, so as to alternately increase and decrease blood pressure in the aorta, in counter phase with the beating of the heart, in order to assist the left ventricle in propelling blood into the arterial system. The Intra-Aortic Balloon (IAB) catheter is a popular cardiac assist device because it can be inserted percutaneously, and therefore, avoids the major surgery associated with implantation and removal of an internal ventricular assist device. The IABP, however, provides only limited augmentation of the heart's natural, unassisted output, and is not adequate for overcoming a major heart failure.
While these devices may be suitable for the particular purpose employed, or for general use, they would not be as suitable for the purposes of the present invention as disclosed hereafter.